1. Field of the Invention
The present invention relates to a charging member and an electrophotographic image forming apparatus.
2. Description of the Related Art
In the charging mechanism (charging principle) of contact charging, two kinds of charging mechanisms of (1) discharge charging mechanism and (2) direct injection charging mechanism are mixed, and the characteristics of a more dominant mechanism thereof appear.
According to the direct injection charging mechanism, charges are directly injected from a contact charging member to a body to be charged, whereby the surface of the body to be charged is charged. A charging apparatus employing a charging brush as the contact charging member is simple in terms of mechanism and is more advantageous in terms of cost than a roller charging method employing a charging roller. Therefore, the charging apparatus is being put into practical use.
However, according to the charging method including performing contact injection charging, sufficient properties in terms of injection efficiency and the like are still difficult to obtain.
In charging with a charging brush, the brush hair of the charging brush needs to be brought into uniform contact with the surface of a photosensitive member. Therefore, as fibers constituting the brush hair, electro conductive fibers in which an electro conductive filler, such as carbon, is dispersed in a base resin, such as Nylon-6, Nylon-66, Nylon-12, polyethylene terephthalate, polyethylene, and polypropylene, are used.
The electro conductive fiber is manufactured by kneading and melting resin compound pellets containing a base resin and an electro conductive filler with a desired ratio in, for example, an extruder, extruding the molten substance from a nozzle plate, and then cooling and drawing the same.
The surface of the electro conductive fiber manufactured by the above-described method almost contains an electrically insulating base resin.
Therefore, when it is attempted to charge a photosensitive member using a charging brush having the brush hair containing such electro conductive fibers, injection of charges from the charging brush to the photosensitive member is performed only when the electro conductive filler exposed to the surface of the electro conductive fiber directly contacts the photosensitive member. Therefore, there has been a problem in that the charging efficiency has been poor.
Japanese Patent No. 4089122 discloses increasing the electrical charge injection rate in a charging member which charges the surface of a body to be charged by mechanically polishing and/or cutting an electro conductive resin molded product to project a part in the longitudinal direction of a carbon nanotube out of the electro conductive resin molded product.
Moreover, Japanese Patent Laid-Open No. 2007-34196 discloses that, by the use of electro conductive fibers in which carbon nanotubes as an electro conductive filler dispersed in a base resin are almost oriented to the longitudinal direction of the fiber, a variation in the electrical resistance values of the electro conductive fibers can be reduced in a charging member for an image forming apparatus.
Moreover, Japanese Patent Laid-Open No. 2008-138304 discloses that, in order to remove a skin layer on the surface of an electro conductive fiber in which carbon nanofibers are dispersed in a polyester resin, the electro conductive fiber is immersed in an aqueous alkaline solution to perform etching treatment of dissolving a resin portion of the fiber surface to thereby reduce the contact resistance of the electro conductive fiber surface.
However, under the injection charging conditions of an increase in speed or an increase in image quality desired in an image forming apparatus, it cannot be expected to give a sufficient charging voltage to the surface of a photosensitive member for reasons described later.
In the charging member disclosed in Japanese Patent No. 4089122, all or some of the carbon nanotubes project from the charging member surface, and therefore, when charges are injected to a photosensitive member, charge are injected only from the tip portion of the carbon nanotubes directly contacting the photosensitive member. Therefore, highly efficient electrical charge injection cannot be expected.
In the electro conductive fiber disclosed in Japanese Patent Laid-Open No. 2007-34196, the direction of the carbon nanotubes dispersed in the base resin are almost uniform in the length direction of the fiber in the base resin. Therefore, when charges are injected to a photosensitive member, a skin layer having electrically high resistance present on the surface of the electro conductive fiber contacts the photosensitive member. Therefore, the injection of charges from the side surface of the electro conductive fiber is not almost performed, so that highly efficient electrical charge injection cannot be expected.
In the electro conductive fiber disclosed in Japanese Patent Laid-Open No. 2008-138304, the skin layer on the fiber surface is removed by chemical etching treatment. However, carbon nanofibers as an electro conductive filler are uniformly arranged in the length direction of the fiber in the polyester resin manufactured by melt spinning method, and therefore when charges are injected to a photosensitive member, charges are injected to the surface of the photosensitive member only in a portion of the carbon nanofibers present on the surface at the side surface of the electro conductive fiber, so that highly efficient electrical charge injection cannot be expected. Furthermore, the chemical etching treatment can be substantially performed only to polyester as a base resin, and is difficult to apply to another resin.